What loses heat fastest, a white or black surface?

Does a black object emit thermal radiation more readily than a white object?
04 September 2011

Question

A black object will absorb thermal radiation more readily than a white object. So does a black object emit thermal radiation more readily than a white object?

Dave - To really understand what's going on here, you want to understand how light is emitted and absorbed. Essentially, when light is absorbed, a photon of light comes in and hits an electron, giving it some energy.

Now, whether it's absorbed or not will depend on the structure of the electrons [in the atom]. If there's somewhere for the electron to go, and if it can increase it's energy level, then it will absorb the light. If not, then the light can't be absorbed and carries straight on.

If it is absorbed, the substance looks black. If it isn't, it's white [because the light is reflected back].

This means that, if you heat something up, if there's [a higher energy level] somewhere for the electrons to go up to, and it's black, then there's also somewhere for the electron to drop down to again, meaning that the substance will also emit energy.

So if something is good at absorbing light then it's also good at emitting it. So a black thing should be much better at emitting light than a white thing.

Chris - So if you have solar panels on your roof, for example, that are heating water, you don't want the water going through them when it's colder outside than the water is because you're turning them into very effective giant radiators?

Dave - That's right. And if you want to cool down a spaceship, the only way of doing it is by radiation of infrared light, so you paint the outside of your spaceship, or the bits of the radiators, in really dark black, which is so much better at emitting the light a much better radiator.

Chris - Alright; slight twist to the story: if you're a cricketer and your at Lords, assuming it is sunny on the occasion you're over there - not that we've been having much sun lately - they wear white, ostensibly to reflect the heat back off. So would they be better off wearing black then, assuming they're going to get hot anyway, so they can radiate the light and the heat better with the black surface?

Dave - They're very, very unlikely to be hot enough to be radiating more heat than the sun is putting down on them because the sun has got a kilowatt per square metre and you're going to have to be very, very warm before you're emitting that much heat. So, I think you're still better off being in white.